Unreachable doesn’t mean it is unusable. It just states that it has missed a request at some time for whatever reason. Mostly the device is working fine. You can try that by using the “Test” function behind the 3 dots on the end. Or, in case of a plug, send a “basic on (or off)” command. In 99.9% of the time it will switch immediately.
If the device is marked “unknown” in the “Route” column it is really unusable.
This is what Athom replied when i asked:
